1. Field of the Invention
The present invention relates to an automotive brake system capable of actively adjusting vehicle stability by a computer-controlled braking force, and specifically to a computer-controlled automotive braking system equipped with at least a skid control unit involving a hydraulic pump for suppressing a so-called deceleration slip which may often occur during braking on low-.mu. roads, and a traction and vehicle dynamics control unit involving another hydraulic pump for suppressing a so-called acceleration slip which may often occur when rapidly accelerating and/or for actively producing a proper yawing moment necessary to improve vehicle stability or vehicle dynamics (e.g., cornering stability or steer characteristics) by virtue of a computer-controlled wheel-brake cylinder pressure (or a computer-controlled braking force).
2. Description of the Prior Art
Recently, there have been proposed and developed various computer-controlled brake systems equipped with a skid control unit (often called an "ABS unit" or an "ABS hydraulic unit") which is designed to prevent skidding and thus provide maximum effective braking, and/or a traction and vehicle dynamics control unit substantially corresponding to a "traction-and-yaw control unit". In such a conventional computer-controlled brake system having an ABS unit and a traction-and-yaw control unit, the ABS unit includes a plurality of fluid pressure control valves and a return pump (an ABS pump) for skid control. On the other hand, the traction-and-yaw control unit generally includes a brake-fluid pressure apply pump serving to rise a wheel-brake cylinder pressure during the traction control or during the vehicle dynamics control (yaw control). On more later-model hydraulic brake system with an ABS unit and a traction-and-yaw control hydraulic unit, a brake-fluid pressure apply piston is further disposed between the brake-fluid pressure apply pump and each brake circuit of a dual-brake system to indirectly supply the brake-fluid pressure via the pressure apply piston unit. One such computer-controlled brake system, which is applicable to skid control and vehicle stability control, has been disclosed in an international application No. PCT/DE93/00802, filed Sep. 4, 1993 (PCT publication No. WO 94/07720, published Apr. 14, 1994), corresponding to Japanese patent publication No. 7-501506, published Feb. 16, 1995. The hydraulic brake system disclosed in the international patent application No. PCT/DE93/00802, has a plurality of fluid pressure control valves fluidly disposed in the hydraulic brake circuit and comprised of inflow valves and outflow valves, and an ABS unit having a return pump (or an ABS pump) which is driven during skid control to brake fluid drained from the outflow valves toward upstream of the fluid pressure control valves (that is, toward the master-cylinder side). Additionally, a suction circuit is connected via a normally-closed inflow gate valve to the suction side of the return pump. A normally-open outflow gate valve is disposed upstream of the discharge outlet of the return pump. Also, the prior art brake system disclosed in the international patent application No. PCT/DE93/00802, has a pressure apply piston in the brake circuit upstream of the outflow gate valve, and a pressure apply pump which sucks brake fluid from the reservoir of the master cylinder and supplies the brake fluid into the suction side of the return pump during the traction-and-yaw control. The pressure apply piston unit has a piston which is slidably accommodated in a piston cylinder and divides an internal space of the cylinder into a primary chamber and a secondary chamber. The primary chamber communicates with the discharge port of the pressure apply pump, whereas the secondary chamber is connected to the suction circuit. The pressure apply piston unit further comprises a communication passageway intercommunicating the upstream side and downstream side of the brake circuit. In the previously-noted conventional brake system, when the depression of the brake pedal produces the master-cylinder pressure, the master-cylinder pressure is transmitted via the communication passageway of the pressure apply piston unit and then transmitted through the brake circuit to the wheel-brake cylinder. This produces a braking force. During braking, if there is a greatly increased tendency of wheel lock, the skid control is initiated by means of the ABS unit. During the skid control, the return pump (ABS pump) is driven and simultaneously the fluid-pressure control valves are operated according to a proper skid control cycle, namely a pressure-reduction mode, a pressure-hold mode, and a pressure build-up mode, in order to prevent undesired wheel lock. When the vehicle excessive understeer or excessive oversteer during cornering, the computer-controlled brake system executes the vehicle dynamics control, to generate a yawing moment counteracting the undesired vehicle behavior or undesired yaw rotation about the z-axis by producing a braking force at each of properly selected road wheels. In the conventional brake system, during the vehicle dynamics control, the outflow gate valve is fully closed, whereas the inflow gate valve is fully opened. At the same time, the return pump and the pressure apply pump are both driven. Thus, the brake fluid in the reservoir located on the master cylinder is supplied to the primary chamber of the pressure apply piston unit by way of the pressure apply pump, and thus the piston of the pressure apply piston unit slides axially, and applies pressure to the brake fluid in the secondary chamber. This pressure forces the brake fluid in the secondary chamber into the suction circuit of the return pump. Then, the return pump sucks the brake fluid supplied into the suction circuit and then discharges it into the brake circuit. The respective fluid-pressure control valve is provided to properly regulate the brake-fluid pressure pumped from the return pump and to feed the regulated brake-fluid pressure to the associated wheel-brake cylinder. The above-noted conventional brake system suffers from the following drawbacks. That is, during the vehicle dynamics control, a proper brake-fluid pressure must be fed to the wheel-brake cylinder for an excessively brief time. For this reason, the conventional brake system is designed so that the brake fluid is fed into the suction side of the return pump by way of the pumping action of the pressure apply pump. By virtue of the pressure-apply action of the pressure apply pump and piston unit, the brake fluid can be discharged toward the wheel-brake cylinder for an excessively short time. This enhances the responsiveness of the computer-controlled brake system. However, in the conventional brake system, a pressure apply pump, a return pump, an inflow gate valve, and outflow gate valve, and fluid-pressure control valves are formed integral with each other in the same housing and constructed as a unit. In order for the pressure apply pump to suck brake fluid from the reservoir of the master cylinder for a brief time, the conventional system has a considerably reduced, limited lay-out flexibility, because such a hydraulic unit must be located at a position as close to the master cylinder as possible, to provide a smooth pressure-apply action and to ensure a high responsiveness of the system operation. On the contrary, in due consideration of mutual installation relationship between such a hydraulic unit and other devices/units in the engine room, it is desirable that this hydraulic unit is located away from the reservoir of the master cylinder. In this case, it takes a long time until the pressure apply pump sucks a required amount of brake fluid necessary for a proper pressure apply action and pumps the brake fluid into the inlet side of the return pump. This reduces the responsiveness of the system operation. Such a reduced system responsibility is remarkable during cold weathers. As discussed above, in the conventional system, it is difficult to balance these two contradictory requirements, that is, a high responsiveness in the system and an enhanced lay-out flexibility. To balance the two contradictory requirements, it is possible to install only the pressure apply pump and piston unit near the master cylinder as a separated hydraulic unit. This increases the number of component parts or the number of units, and thus results in the increase in production costs. Also, this lowers the efficiency of installation of the hydraulic unit on the vehicle and thus reduces the productivity of the brake system.